The St. Louis auteur contributed three things to rock music that no one else did: (1) An irresistible swagger, (2) a focus on the guitar riff as the primary melodic element and (3) an emphasis on songwriting as storytelling.
1) In terms of the aforementioned swagger, Berry injected a cocksure 'we know better than the adults' attitude into rock -- something his predecessors and peers hadn't yet dared to do. That youth-privileging outlook was essential in transforming rock n' roll from a musical fad into an irresistible attitude and lifestyle that infected teens and spread across America (also, it arguably paved the way for the massive generational divide of the '60s).
2) As for the focus on the guitar riff, compare Berry to his peers. The 'star' element in Elvis' Sun Sessions recordings was the performance; the best part of a Fats Domino single was the sing-able chorus and shuffling melody; Little Richard's "Tutti Frutti," while essential to the formation of rock, is an astonishing work of art because of his performance -- not because of the notes in the composition.
But Berry, however, crafted material where the riff was king. With Elvis or Richard, it's the singing that gets stuck in your brain, but with Berry, it's the guitar riff that gets played over and over in your head. It was Berry's introduction of undeniable guitar hooks into rock that steered the genre away from the ivory tickling of people like Little Richard, Fats Domino and Jerry Lee Lewis and toward simple guitar chords that were easy for teenage fans to imitate -- guitars are cheaper than pianos and easier to learn -- and impossible to get out of your head. And that was, more or less, all Berry (Bo Diddley focused on the guitar riff as well, but his otherworldly tuning techniques and sonic palettes were far stranger than Berry's; he opened up rock to a more experimental future, but his songs were too esoteric to have the same impact and ubiquity as Berry's more affable material).
Ultimately, that focus on the riff was what distinguished rock from pop and R&B and what changed the course of popular music for decades to come. From the Rolling Stones to Jimi Hendrix to Led Zeppelin to the Ramones, it's that guitar riff fetish that defines rock as a distinct musical form. And from about 1955-1960, Berry unleashed one unstoppable riff into the world after another via a series of iconic singles (for the newcomer, The Great Twenty-Eight collection is the best starting point).
3) As for his songwriting, Berry eschewed generic emotional confessions and instead focused on crafting short stories with his lyrics. His songwriting style -- economical, vivid and enveloping -- influenced everyone from Paul McCartney to Ray Davies to Brian Wilson and set the course for rock to favor the short and sweet instead of the poetic and verbose. That's just not something his contemporaries were pioneering -- Little Richard's lyrics were brilliant nonsense, Bo Diddley's were stream of consciousness poetry, and Elvis Presley didn't write his own material. So Berry's introduction of storytelling into rock can't be overstated, particularly since that's what helped the genre stand out from straight pop in its first few decades.
There's plenty more that can, and will, be said about Berry as the world reflects on his legacy following his death. But one thing is undeniable -- there are few humans whose impact on the world was as unlikely and as far-reaching as Chuck Berry's.